Transposable elements (TEs) and DNA repeats are commonly targeted by DNA and histone methylation to achieve epigenetic gene silencing. We isolated mutations in two Arabidopsis genes, AtMORC1 and AtMORC6, which cause de-repression of DNA-methylated genes and TEs, but no losses of DNA or histone methylation. AtMORC1 and AtMORC6 are members of the conserved Microrchidia (MORC) adenosine triphosphatase (ATPase) family, predicted to catalyze alterations in chromosome superstructure. The atmorc1 and atmorc6 mutants show decondensation of pericentromeric heterochromatin, increased interaction of pericentromeric regions with the rest of the genome, and transcriptional defects that are largely restricted to loci residing in pericentromeric regions. Knockdown of the single MORC homolog in Caenorhabditis elegans also impairs transgene silencing. We propose that the MORC ATPases are conserved regulators of gene silencing in eukaryotes.

The precise temporal and spatial coordination of histone lysine methylation dynamics across the epigenome regulates virtually all DNA-templated processes. A large number of histone lysine methyltransferase (KMT) enzymes catalyze the various lysine methylation events decorating the core histone proteins. Mutations, genetic translocations and altered gene expression involving these KMTs are frequently observed in cancer, developmental disorders and other pathologies. Therapeutic compounds targeting specific KMTs are currently being tested in the clinic, although overall drug discovery in the field is relatively under-developed. Here we review the biochemical and biological activities of histone KMTs and their connections to human diseases, focusing on cancer. We also discuss the scientific and clinical challenges and opportunities in studying KMTs.An important mechanism for regulating chromatin involves the reversible covalent modification of histones by chemical moieties such as methyl and acetyl groups. These different chemical 'marks' on histones are linked to discrete chromatin states and regulate the accessibility of DNA to trans-acting factors that mediate a wide variety of chromatintemplated processes such as transcription, DNA repair and DNA replication 1 . Chemically, lysine methylation entails the addition of one, two or three methyl groups to the ε-nitrogen of a lysine side chain, forming mono-, di-and trimethylated derivatives (referred to here as Kme1, Kme2, and Kme3, respectively; Fig. 1a). This reaction, while only subtly changing the primary structure of the modified polypeptide, greatly increases the information encoded within the molecule, a feature highlighted by the unique activities frequently coupled to the specific extent of methylation. Methylation of lysines on histone and non-histone proteins is generated by protein lysine methyltransferases (KMTs; referred to as 'writers') and removed by protein lysine demethylases (KDMs; referred to as 'erasers') (Fig. 1a). In the human genome, there are predicted to be over 100 KMTs, and mass spectrometry-based studies suggest that more than 1,000 proteins in the human proteome harbor lysine methylation 2-4 . Lysine methylation was first described in 1959 on a bacterial flagellar protein 5 and soon thereafter identified on histone proteins 6 . Indeed, the core histones contain numerous Reprints and permissions information is available at www.nature.com/reprints.

Understanding the causal link between epigenetic marks and gene regulation remains a central question in chromatin biology. To edit the epigenome we developed the FIRE-Cas9 system for rapid and reversible recruitment of endogenous chromatin regulators to specific genomic loci. We enhanced the dCas9–MS2 anchor for genome targeting with Fkbp/Frb dimerizing fusion proteins to allow chemical-induced proximity of a desired chromatin regulator. We find that mSWI/SNF (BAF) complex recruitment is sufficient to oppose Polycomb within minutes, leading to activation of bivalent gene transcription in mouse embryonic stem cells. Furthermore, Hp1/Suv39h1 heterochromatin complex recruitment to active promoters deposits H3K9me3 domains, resulting in gene silencing that can be reversed upon washout of the chemical dimerizer. This inducible recruitment strategy provides precise kinetic information to model epigenetic memory and plasticity. It is broadly applicable to mechanistic studies of chromatin in mammalian cells and is particularly suited to the analysis of endogenous multi-subunit chromatin regulator complexes.

Epigenetic gene silencing is of central importance to maintain genome integrity and is mediated by an elaborate interplay between DNA methylation, histone posttranslational modifications, and chromatin remodeling complexes. DNA methylation and repressive histone marks usually correlate with transcriptionally silent heterochromatin, however there are exceptions to this relationship. In Arabidopsis, mutation of Morpheus Molecule 1 (MOM1) causes transcriptional derepression of heterochromatin independently of changes in DNA methylation. More recently, two Arabidopsis homologues of mouse microrchidia (MORC) genes have also been implicated in gene silencing and heterochromatin condensation without altering genome-wide DNA methylation patterns. In this study, we show that Arabidopsis microrchidia (AtMORC6) physically interacts with AtMORC1 and with its close homologue, AtMORC2, in two mutually exclusive protein complexes. RNA-sequencing analyses of high-order mutants indicate that AtMORC1 and AtMORC2 act redundantly to repress a common set of loci. We also examined genetic interactions between AtMORC6 and MOM1 pathways. Although AtMORC6 and MOM1 control the silencing of a very similar set of genomic loci, we observed synergistic transcriptional regulation in the mom1/atmorc6 double mutant, suggesting that these epigenetic regulators act mainly by different silencing mechanisms.epigenetics | plant biology